How does Idle Air Control Valve Work?

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bkfitz

10 Year
Joined
Jan 7, 2013
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65
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371
Location
Columbia, SC
Still trying to identify my high idle problem... IAC and TPS check out per FSM, but I still feel like my high idle may be related to the IAC getting stuck open (thinking high idle must be due to too much air)...

Question: What events trigger changes in the IAC? And do the triggers result in opening or closing IAC valve?
 
inkpot said:
Any chance you have a random vacuum leak in all the smog and miscellaneous hoses that run everywhere?
Click to expand...

I replaced every ~ 1/2" vacuum line on the truck and several of the larger ones. Dont notice any leaks on the huge intake one bit who knows...
 
Sticking dash pot?
 
pappy said:
Sticking dash pot?
Click to expand...

Is that the "choke" idle up knob on the dash? If so... good thought to check that.

Update: checked it inside the cab and it doesn't seem to help... Is there a place under the hood that I should check to make sure releasing okay?
 
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No, it's a gizmo off the throttle body that keeps the idle up briefly when you let off the gas.
 
pappy said:
No, it's a gizmo off the throttle body that keeps the idle up briefly when you let off the gas.
Click to expand...

Where on the throttle body is the "dash pot" (does it have a more formal name?)

Also I just ran diagnostics (I had previously checked it about 2 days of running after a reset and it pulled nothing)... this time it pulled code 24 (air temperature sensor). Could this be the culprit? Anywhere I can read up on replacement/testing? Looked at FSM testing for "air flow meter", but it doesn't reference the temperature sensor.
 
Looks like the dashpot sits behind the throttle lever... like a shock for the throttle for when you take foot off gas...

...my high idle happens right off the bat... not after releasing gas... so could it still be the culprit?
 
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Where is the adjustment screw on the throttle body? Have you turned it all the way down?

Have you checked for vacuum leaks, including the rubber hoses on the intake between the AFM and the TB?

The idle valve screws onto this base thingy that gets carboned up. Did you clean it?
 
Where is the idle adjustment screw on the throttle body? Have you turned it all the way down?

Have you checked for vacuum leaks, including the rubber hoses on the intake between the AFM and the TB?

The idle valve screws onto this base thingy that gets carboned up. Did you clean it?

Is the throttle cable too tight? Have you tried running the idle with the cable off the TB? Is the TB sticking? Try forcing the TB closed. Dirty TB?

Look for other sources of vacuum leak, especially in places you would not normally think of. Like the dipstick, rocker arm cover gasket, semi-circle plugs on the ends of the head, leaking PCV grommet, worn oil cap gasket, etc.
 
bkfitz said:
Where on the throttle body is the "dash pot" (does it have a more formal name?)

Also I just ran diagnostics (I had previously checked it about 2 days of running after a reset and it pulled nothing)... this time it pulled code 24 (air temperature sensor). Could this be the culprit? Anywhere I can read up on replacement/testing? Looked at FSM testing for "air flow meter", but it doesn't reference the temperature sensor.
Click to expand...
I believe the air temperature sensor is built into the air flow meter.
 
Lead Head said:
I believe the air temperature sensor is built into the air flow meter.
Click to expand...

Yeah... FSM shows it inside the air flow meter... I chexkwd oit the AFM last nihht per FSM and all is well... Just wondering if the air temperature sensor could cause a huge difference in idle speed
 
Try putting small LED lights (noid lights) across the windings of the isc- they'll tell you what the computer's trying to do with the idle.
If it's sticking, you'll see them cycle slowly and contimuously as the truck tries to idle itself down.

You'll be surprised how much it moves around in daily driving...

t
 
pappy said:
Where is the idle adjustment screw on the throttle body? Have you turned it all the way down?

Have you checked for vacuum leaks, including the rubber hoses on the intake between the AFM and the TB?

The idle valve screws onto this base thingy that gets carboned up. Did you clean it?

Is the throttle cable too tight? Have you tried running the idle with the cable off the TB? Is the TB sticking? Try forcing the TB closed. Dirty TB?

Look for other sources of vacuum leak, especially in places you would not normally think of. Like the dipstick, rocker arm cover gasket, semi-circle plugs on the ends of the head, leaking PCV grommet, worn oil cap gasket, etc.
Click to expand...

- Idle screw is turned all the way down (lowest possible idle)
- Throttle cable is loose
- Throttle body itself was just completely cleaned, new gasket, etc
- Throttle body piece behind IAC was just cleaned
- Replaced every ~1/2" vacuum line
- TPS checks out per FSM and is turned to result in the lowest idle
- IAC checks out per FSM
- Air flow meter checks out per FSM although I'm still getting code 24 (air temp sensor)

Still idling between 900-1200 depending on day/weather/etc

+ Haven't noticed any leaks in the intake, but I'll take another look
+ How do I 'force the TB closed?'
+ Is there a preferred method for finding vacuum leaks in hard to tell places like the dipstick etc?

I'm thinking it's time to take her in to an expert to check/set the timing, check the IAC with noid lights, cold start injectors, etc... anything else you would put on the list for the expert to check?

Thanks!!!!
 
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To check for vaccum leaks in the intake, I take a little starting fluid and spray around. If you get an increase in idle...track the leak. Just use common sense, it is starting fluid.

Sometimes the spring that closes the vane in the throttle body will wear and it will not close tight up and down. Thus always letting in air and a high idle. Just see if you have to manually force the vane closed.

How did you check the IAC? Those usually just need a good cleaning with some picks. It seems to be a catch for old RTV in the cooling system to end up.
 
snobdds said:
To check for vaccum leaks in the intake, I take a little starting fluid and spray around. If you get an increase in idle...track the leak. Just use common sense, it is starting fluid.

Sometimes the spring that closes the vane in the throttle body will wear and it will not close tight up and down. Thus always letting in air and a high idle. Just see if you have to manually force the vane closed.

How did you check the IAC? Those usually just need a good cleaning with some picks. It seems to be a catch for old RTV in the cooling system to end up.
Click to expand...

I'll try the starting fluid spray... thx for the recommendation.

I checked the IAC with an ohmmeter per FSM resistances/impedances. I also cleaned the part of the throttle body it connects to. Other than wiping off any carbon tar, is there anything else I can do to clean the IAC. See pic below:

I think the throttle body spring is ok... see pic below:

Before:

20130406_160937.jpg


After:

20130406_174415.jpg


IAC Testing:

20130622_093445.jpg


20130622_093655.jpg
 
That is the dirtiest throttle body I have ever seen. Good job on cleaning it.

I would clean out all the passages that the coolant lines connect to. RTV and other junk in the cooling system gets stuck in there and will mess with the idle. See below:

getimage.php
 
snobdds said:
That is the dirtiest throttle body I have ever seen. Good job on cleaning it.

I would clean out all the passages that the coolant lines connect to. RTV and other junk in the cooling system gets stuck in there and will mess with the idle. See below:
Click to expand...

Oh yeah... cleaned that iac throttle piece equally well including input outputs and replaced all lines in out of it.
 
although I'm still getting code 24 (air temp sensor)
Click to expand...

Did you meter the air temp sensor? It has a pin (2 pins?) in the AFM harness.

If it meters OK, I'd make sure it's getting its signal back to the ECU.

That WILL do odd things to idle if the truck thinks it's -40 or +130....


But given all that goo, jeez, I'd really confirm that passages aren't clogged up
inside there.

hth

t
 
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